1. Field of Invention
This invention relates to a method and apparatus for mixing gases in precise proportions. Such a device is especially useful for maintaining a controlled composition atmosphere in devices such as laboratory incubators.
2. Description of the Prior Art
Several laboratory procedures require the use of a controlled composition atmosphere. There are numerous known methods and associated devices for precisely mixing gases to obtain such an atmosphere. In one system, a gas composition sensing device, such as a thermal conductivity sensor, senses gas composition in the chamber where it is desired to maintain a precisely controlled atmosphere. When the sensor device detects that the concentration of a component gas (e.g., CO.sub.2) has dropped below a predetermined threshold, it opens a solenoid valve to permit more of that gas to flow into the chamber. This gas continues to flow until the sensing device indicates that the desired concentration has been restored, at which time it causes the solenoid valve to close. In such a system, gas does not flow continuously, but rather is introduced into the chamber only as required. The reliability and accuracy of such a system is necessarily limited to the reliability and accuracy of the gas composition sensing device employed.
In another known system, the gases to be mixed continuously flow together at a confluence upstream of the chamber. The desired proportion is achieved by using a flow regulation device to limit the flow of each gas to a desired known rate prior to its confluence with other gases. Thus, in such a system, the relative mixing ratios are achieved by preselecting the relative flow ratios of the separate gases. The mixing ratio is usually fixed. After confluence, the mixed gas stream flows at a constant rate of flow into the chamber in which the controlled composition atmosphere is to be maintained. The mixed gas stream is slightly pressurized with respect to the atmosphere in the chamber so that it forces gas already in the chamber to exhaust through leaks provided for that purpose.
In either of the systems just described, provision is commonly made for purging systems which quickly re-establish the controlled composition atmosphere in the chamber after contamination with outside air.
A third known method employs solenoid valves to introduce the gases to be mixed separately and sequentially into the chamber. The desired mixing ratio is achieved by varying the proportional amount of time each gas is permitted to flow into the chamber. Such a system is disclosed in U.S. Pat. No. 4,062,373 granted to Clark et al. on Dec. 13, 1977. The system disclosed there, however, requires a sophisticated technique for balancing the pressure in the separate gas lines upstream of their respective solenoid valves to assure equal rates of flow for each gas for the time interval it is permitted to enter the chamber. Also, the system makes no provision for purging unwanted gas and rapidly restoring the desired atmosphere after contamination.
These and possibly other types of prior art gas mixing systems are disclosed in the U.S. patents listed below:
U.S. Pat. No. 4,062,373--Clark et al. PA1 U.S. Pat. No. 4,019,523--Clark et al. PA1 U.S. Pat. No. 3,830,256--Cox PA1 U.S. Pat. No. 3,367,308--Quattrone et al. PA1 U.S. Pat. No. 3,749,111--Dobritz PA1 U.S. Pat. No. 3,809,109--Breiling et al. PA1 U.S. Pat. No. 3,848,617--Dray PA1 U.S. Pat. No. 3,886,971--Lundsgaard et al. PA1 U.S. Pat. No. 3,905,384--Berger PA1 U.S. Pat. No. 4,072,148--Munson et al.